I can speak to building a stronger deadlift, which incidentally has made me more muscular. It doesn't sound like that's what you're really after though.
I don't have any experience in training strictly for muscle size, so I can't speak to that. As with most things training-related, I'd look to those who have achieved what you want to achieve. There are lots of strong and muscular 45+ people here and elsewhere who've laid their methods out for us all to see. Success leaves tracks, so follow them.
That said, I'd encourage you to think about what you want and what you are willing to do to get it. I don't mean any disrespect, but it sounds like your goals are poorly-defined.
You first stated that you wanted to...
There is nothing wrong with that, but when I suggested looking into a serious strength program, you changed your tune to...
Once again there's absolutely nothing wrong with that, but you might approach that goal differently than a strength-focused one.
If you want to add size, I'd recommend choosing a proven bodybuilding program that you can fit into your life and running it as well as you can. The clock is ticking for everyone in this section, and our bodies will never be in a better position to add muscle than they are today.
Of course, if that sounds like too much hassle, there's nothing wrong with not doing any of that crap. All of this stuff is optional. Maybe you just want to keep doing what you've been doing, but with some deadlifts added.
If that's the case, just add them wherever you feel makes the most sense and see how your body responds. I've done deads the same day as heavy squats and made out just fine, or you can do them two days after squats, or on your back/chest day or delts/arms day or even a day where you just go in and do some deadlifts and nothing else. In this context, I don't think it matters all that much if your recovery is on-point.
There are so many ways you can approach this, as long as you are being sensible I don't think you can really go too wrong.